Conventionally, various processing methods for wood have been proposed. As one type of processing method, the applicant has proposed a corrugated board acquired by processing a board in a corrugated shape and a method for manufacturing the corrugated board (refer to Patent Literature 1). This method is performed such that a board is fed between two heat rollers each provided in its outer periphery with asperities to be pressed and curved by the two heat rollers to acquire a corrugated board.
This method allows the applicant to succeed in acquiring a corrugated board formed in a corrugated shape.
Here, a raw material of this corrugated board is a board. Thus, it is preferable not to apply coating or the like to a surface of the board to utilize a texture and an aroma of the board. When coating or the like is not applied to a board after being processed in a corrugated shape, a flexural portion is loosened due to properties of wood to cause a force for gradually restoring the board to its original straight shape. Particularly, when a board is placed in a humid environment or when it is wetted by moisture, the force for restoring it increases to make it difficult to maintain a curvature rate at the beginning of processing.
Meanwhile, other methods each also have been proposed as a method for processing a board in a corrugated shape (refer to Patent Literatures 2 to 4). These methods each not only allow a board to be processed in a corrugated shape with a suitable jig, but also have a devised way such as using plywood, or attaching separate boards on respective sides of a board with an adhesive. When plywood in which boards are laminated in multiple layers while being bonded with an adhesive is used, or boards are attached on respective sides of a board with an adhesive, as described above, a corrugated state of a corrugated board is maintained by the adhesive. This enables the corrugated board to be prevented from returning to a board shape even in a place with high humidity or even when being wet with water.
However, this kind of method always needs a material other than wood such as adhesive to maintain a flexural state of the wood, so that there are problems that it is difficult to utilize an aroma and a texture of the board, and that a material other than natural substances (wood), such as a synthetic adhesive, needs to be used.